Structural studies of simple viruses are an important way to explore general mechanisms in the assembly of macromolecular components and the regulation thereby of cellular growth and differentiation. This proposal outlines the analysis at high resolution of several related RNA viruses - tomato bushy stunt virus (TBSV), turnip crinkle virus (TCV), and exapnded forms of these particles - by the use of X-ray diffraction and allied techniques. The successful visualization at 2.9 A resolution of TBSV has revealed a number of remarkable features of the organization of the virus particle and of its subunit. Distinct functions in assembly appear to be properties of distinct domains. Continued structural studies of TBSV and TCV will add detail to the present picture, define structural changes that occur during reversible particle expansion (apparently regulated by divalent cation binding), and probe current ideas about RNA-protein interactions (which seem to be mediated by a flexibly-linked N-terminal arm of the protein subunit). Proteolytic dissection of TBSV and TCV subunits into constituent domains is proposed as part of an initial approach to biochemical study of their functional properties. Physico-chemical studies of subunit association in solution and of the mechanism of expansion are also described, as first steps in defining an assembly pathway.